The waste material from a textile treatment plant contains large amounts of various bleaches, dye waste liquors, miscellaneous organic salts, and suspended solids, including textile fibers. Moreover, the waste material is typically at elevated temperatures utilized in textile treatment processes.
The discharge of such waste into municipal sewers or nearby rivers would have numerous undesirable consequences. It is desirable to treat the waste so as to remove pollutants, recover salt and heat contained in the raw effluent, and produce processed water suitable for reuse in the plant.
By treatment of the plant waste in this fashion, large volumes of impure water are prevented from polluting the surrounding water environment. If the water is recycled, minimum effluents are discharged; if the water is not recycled the effluents are basically non-polluting. In either event, the plant will have succeeded in fulfilling a basic civic duty of avoiding pollution while insuring its compliance with relevant state and federal environmental protection regulations.
Osmosis is a known process involving migration of liquids through a semipermeable membrane. In particular, when liquids containing different concentrations of a solute are on opposite sides of the membrane, a pressure called osmotic pressure exists, causing liquid to pass from the region of dilute solution, through the membrane and into the region of concentrated solution, thereby diluting the more concentrated liquid and effecting an equalization of the osmotic pressures on each side of the membrane. The normal flow of liquids through the membrane may be stopped by the application of pressure to the solution on the higher concentration side. The pressure required is known as the osmotic pressure and is a characteristic of the particular solute/solvent system involved.
The application of a pressure in excess of the osmotic pressure to the region of concentrated solution is known to cause liquid and other permeates to flow to the region of dilute solution, thereby increasing the inequality of concentration. Such a procedure is named "reverse osmosis." By employing a series of reverse osmosis steps, with appropriate pressure application, relatively pure liquid can be recovered.
However, reverse osmosis tends not to work if the increased concentration on the high pressure side leads to precipitation of solids. In particular, if the solution is already in effect saturated, treatment in a reverse osmosis unit would result in precipitation and clogging up of the membrane. U.S. Pat. No. 4,046,686 discloses the addition of a sequestering agent to prevent such precipitation.